Even as climate policy is debated in Doha, it's becoming increasingly clear that the first devastating effects of global warming cannot be prevented.

Amid the glittering skyscrapers of Doha, capital of the arid, oil-rich Arab emirate of Qatar, 17,000 diplomats, delegates, nongovernmental organizations, and environmentalists are converging this week and next in the conference halls and backrooms of the 18th annual United Nations climate-change summit. Their goal: pave the way toward a world treaty, to be signed in 2015, aimed at slowing global emissions of heat-trapping fossil-fuel pollution enough to keep the planet’s temperature from rising by 2 degrees Celsius (3.6 degrees Fahrenheit).

That’s the point, scientists say, at which the Earth’s polar ice sheets will melt and many of the hottest and driest regions will no longer be able to grow food. The 2-degree mark will set off a chain of extreme reactions, starting with rapid sea-level rise, widespread flooding, more extreme weather events, food shortages, and price spikes.

But no matter what the diplomats in Doha decide over the next week, it now appears inevitable that the world will indeed hit that 2-degree mark and could well shoot past it to average global increases of 4 degrees or 6 degrees—points at which scientists predict even worse catastrophes.

A scientific study published Sunday in the journal Nature Climate Change concluded that the world’s rapid increase in fossil fuel emissions now makes a global average temperature increase of 2 degrees Celsius all but inevitable. A report released last week by the U.N. Environment Program concluded that given the rapid projected increase in pollution from burning coal, oil, and gas around the world, nations’ current pledges to cut carbon emissions won’t be enough to stave off that 2-degree rise sometime before the end of the century.

Moreover, a November report from the International Energy Agency found that if action isn’t taken to significantly cut carbon emissions by 2017, the existing power plants, factories, and buildings will be enough to push temperatures past the 2-degree mark. Yet another sobering report last month from the accounting firm PricewaterhouseCoopers warned that the only way the world can prevent the 2-degree rise is if the global economy cuts its carbon intensity by 5.1 percent every year from now to 2050, essentially slamming the brakes on growth starting right now—and keeping the freeze on for 37 years.

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Nobody expects that to happen. And even while the U.N. climate-change process is working toward a 2015 global deal in which the world’s biggest polluters agree to cut their carbon pollution, the terms of the agreement won’t be enforced until 2020. That means countries won’t even be required to start cutting their emissions for another eight years.

“The necessary rate of decarbonization is so high that it’s never been seen before, never been done before,” said Jonathan Grant, author of the PricewaterhouseCoopers report. “And the level of ambition that we’re seeing at Doha is far below what is required to stick to 2 degrees.”

So what needs to happen? Nations must begin to prepare now for the effects of a 2-degree temperature increase. In the U.S., that means starting immediately to plan and build for the inevitable consequences of more-destructive storms and rising sea levels, particularly in coastal cities such as New Orleans; Norfolk, Va.; and, as superstorm Sandy so frighteningly illustrated, New York. Those preparations are going to cost taxpayers a lot of money.

“When it comes to the worst-case scenarios of sea-level rise, I’m not sure $100 billion will even scratch the surface,” said Brian Murray, director of economic analysis at Duke University’s Nicholas Institute for Environmental Policy Solutions.

It also means that nations—first and foremost, the world’s two biggest polluters, the United States and China—must develop real, enforceable, and aggressive policies to cut their own carbon emissions, with or without a global agreement. Scientists say that once the world hits that 2-degree mark, the urgency of reducing carbon pollution to avoid a catastrophic tipping point becomes even greater.

Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University and a member of the Nobel Prize-winning U.N. Intergovernmental Panel on Climate Change, says that a 2-degree rise is not itself that point, but rather the beginning of irreversible changes. “It starts to speed you toward a tipping point,” he said. “It’s driving toward a cliff at night with the headlights off. We don’t know when we’ll hit that cliff, but after 2 degrees, we’re going faster, we have less control. After 3, 4, 5 degrees, you spiral out of control, you have even more irreversible change. At this point, with prompt action to reduce emissions, we can still keep it from getting totally out of control.”

When it comes to global climate-change treaties, the U.S. has already cried wolf twice on the world stage. In 1997, Vice President Al Gore urged the rest of the world to sign on to the landmark Kyoto Protocol, but the Senate refused to go along. At the 2009 U.N. climate-change summit in Copenhagen, President Obama took on the same role, urging China and other nations to sign pledges to cut carbon emissions, assuring the world that Congress would soon pass a cap-and-trade bill to cut U.S. carbon emissions. Six months later, the bill died in the Senate.

In December 2015, when it comes time to forge a binding global agreement that could keep the world from heading off a climate cliff, Obama won’t be able to face his counterparts on the world stage with just a promise of what’s to come. To get the world’s other major polluters to agree to cut the carbon pollution that threatens U.S. coastal cities, he’ll need to have a new U.S. climate law in hand.

“It would be very easy, in 2015, for the Obama administration to appear to demonstrate international leadership by agreeing to sign on to something that it knows can’t get ratified by the Senate,” said Robert Stavins, director of the environmental economics program at Harvard University. “But the only way this time will be different is if there’s an agreement that says the U.S. is committing to action on what it’s already done.”